We are currently studying Resonance Raman (RR) spectroscopy of flavins and flavoproteins. The major impediment to such work is the strong fluoresence of flavins; we have dealt with this fluorescence intereference by using chemical quenching agents such as KI or the protein of flavoprotein complexes. We have obtained RR spectra of flavin-metal complexes, N-1 and N-3 ionized and protonated species, covalent derivatives of flavins, flavin semiquinone, and charge transfer complexes. To date it appears that RR spectroscopy is a good diagnostic tool for determining the chemical state of flavin species. We are now working with flavoproteins involved in beta-oxidation of fatty acyl CoA esters in mitochondria. We have obtained a spectrum of general fatty acyl CoA dehydrogenase and the RR spectrum suggests that the N-3 hydrogen and FAD is more strongly hydrogen bonded to protein than it is to the water of aqueous solutions. Furthermore, we have employed RR spectroscopy to characterize a complex of fatty acyl CoA dehydrogenase and acetoacetyl CoA as an excited state charge-transfer complex with little ground state electron transfer. Electron transfer flavoprotein (ETF) semiquinone is being investigated by RR spectroscopy as well as the high potential iron sulfur flavoprotein to which ETF transfers its electrons. In the latter protein we are determining (by RR spectroscopy) whether direct coordination occurs between Fe and flavin. Finally, we are also investigating fatty acyl CoA oxidase from liver peroxisomes in order to compare the flavin environment with that of the dehydrogenase.